Arrangement in microwave stoves



Feb. 20, 1968 L. MALMQUIST 3,370,145

ARRANGEMENT IN MICROWAVE STOVES Filed April 1, 1965 A E ,1 X

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INVENTOR 1.50 MALMOUIST AGEN United States Patent 3,370,145 ARRANGEMENT IN MICROWAVE STOVES Leo Malmqnist, Ektorp, Sweden, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Apr. 1, 1965, Ser. No. 444,548 Claims priority, application Sweden, Apr. 7, 1964, 4,244 5 Claims. (Cl. 21910.55)

The present invention relates to an arrangement in microwave stoves in which a heating cavity is energized by means of a magnetron via a wave guide connection terminating in an opening in one of the walls of the cavity.

In order to function properly, a microwave stove utilizing a magnetron as the energy source requires on the one hand that the magnetron be loaded such that it operates within a favourable working range, and on the other hand that the energy distribution within the stove cavity be uniform. In order to satisfy the last requirement, it is known to arrange within the cavity a periodically movable body, a .so-called stirrer, which influences the field pattern within the cavity. The stirrer may, for example, consist of one or more reflecting disks arranged near to the energy feed opening and fixed in an inclined position to a shaft so that upon rotation of the shaft the said disks are brought successively to a position opposite the energy feed opening. Such a stirrer functions on the one hand to periodically change the resonance frequency of the cavity, and on the other hand to reflect the energy coming through the energy feed opening in different directions within the cavity, which functions both have the effect of making the energy distribution within the cavity more uniform.

An object of the invention is to produce a more uniform energy distribution in a microwave stove of the kind described than has hitherto been possible by means of commonly used stirrers, which is in principle achieved in that the magnetron is influenced by means of a reflected wave such that it periodically changes its frequency.

It is known that a reflected wave which returns to a magnetron in a certain phase position may be favourable in that it increases the output power of the magnetron. The phase of the reflected wave at which this effect is maximum is called the sink phase and the corresponding range in the magnetron loading diagram is called the sink range. When operating the system so that the phase of the reflected wave corresponds to operation in the sink range, however, another phenomena also appears which has not previously been utilized, namely the phenomena that relatively small changes in the phase of the reflected wave give rise to appreciable frequency variations of the .disks situated opposite the energy feed opening in the cavity. The reactive matching element is adjusted in relation to the said periodically variable reactan'ce such that the reflected wave from the matching element in the di rection towards the magnetron, in combination with the reflected wave coming from the variable reactance, gives rise to a resulting wave in direction towards the magnetron which causes the magnetron to work substantially in the sink range over the whole variation range of the 3,370,145 Patented Feb. 20, 1968 ICE variable reactance. In this way it is ensured not only that the magnetron will have an appreciable frequency variation resulting in an even energy distribution in the cavity, but also that the magnetron will always operate with high output power.

The invention will now be described more fully in connection with the accompanying drawing, in which:

FIG. 1a shows a schematic diagram of an arrangement according to the invention,

FIG. 1b shows the field variations in a section of the wave guide in the device according to FIG. 1a (standing wave) and FIG. 2 shows a loading diagram for the magnetron included in the device according to FIG. 1a.

In FIG. 1, reference numeral 1 designates a magnetron which is connected through a wave guide 2 to the heating cavity 3 of a microwave stove. In the wave guide is situated a reactive matching element in the shape of a pin 4 projecting into the wave guide. The pin is displaceable in the length direction of the Wave guide by being arranged in a longitudinal slot in the side wall of the Wave guide. A stirrer in the shape of a rotatable wheel 5 with inclined wings is situated near the energy feed opening in the cavity.

Due to reflections on the one hand .at the reactive ele ment 4, and on the other hand at the stirrer 5, a resulting reflected wave in the direction towards the magnetron is produced which gives rise to a standing wave, for example, as shown in FIG. 1b. The reflected wave influences the magnetron so that its working point will be dependent both upon the phase and the magnitude of the reflected wave. The magnitude of the reflected wave is commonly specified by the standing wave ratio, which is the relation between the maximum and minimum field of the standing wave, i.e. the relation v /v in FIG. 1b. The phase of the reflected wave is commonly determined by measuring the distance between the minimum of the standing wave and a reference plane A-A in the magnetron, i.e. the distance d in FIG. 1b expressed in fractions of the wave length of the working frequency. v

The loading diagram for a magnetron is shown in FIG. 2 and illustrates how the magnetron will respond to different loads resulting in different reflected waves. The input variables in FIG. 2 are on the one hand the said standing wave ratio and on the other hand the distance between the minimum of the standing wave and the reference plane. Loci of points having the same standing wave ratio in FIG. 2 form concentric circles, while loci of points having the same value of the said distance d form radial lines emanating from the centre of the circle diagram. The output magnitudes are the power and frequency of the magnetron. Loci of points having the same out-put power form curves drawn in full lines and designated P, while loci of points having the same frequency form curves drawn in dotted lines and designated A A is the deviation in frequency from the given working frequency.

If the magnetron operates without a reflected wave, that is in exactly matched condition, the working point will be the centre point of the circle diagram in FIG. 2. Its frequency is then equal to the working frequency of the magnetron and the output power is, in the example shown, equal to 2 kilowatts. If there is a mismatch in the feeding connection or at the load so that a reflected wave is produced, this mismatch can, according to the diagram, influence the magnetron in different ways dependent upon the phase position in which the reflected wave arrives. Thus if the reflected wave has such a phase position that the standing wave has its minimum at a distance which in FIG. 2 is designated sink, in the example shown equal to about 0.41 A, where A is the wave length corresponding to the Working frequency, the output power will, according to the diagram, increase with increasing value of the standing wave ratio, i.e. with increased reflected wave. On the other hand, if the phase position of the reflected wave is such that the standing Wave has its minimum at, in the example shown, approximately 0.16 A, which is called antisink, the output power will instead decrease with increasing value of the standing wave ratio. In order to achieve a high output power from the magnetron it is required that the magnetron operate within the sink range and not in the antisink range.

From the diagram it is furthermore evident that the frequency curves in the sink range are closer to each other than in the antisink range. This means that a small variation in the phase position of the reflected wave in the sink range will give rise to a relatively great frequency variation of the magnetron.

According to the invention, this is utilized so that, by means of the matching element 4 in combination with the stirrer 5, it is ensured that the magnetron operates substantially within the sink range at all positions of the stirrer 5. During operation, the stirrer will then produce the small variations in the phase position of the reflected wave which are required for achieving the frequency variation of the magnetron and thereby a more uniform energy distribution in the stove. The matching element 4 is adjusted during variation of the reactance 5 such that the working points of the magnetron at the different positions of the reactance 5 will fall within a range lying near to the line designated sink in FIG. 2. Experiments have shown that a change of the useful load within the stove will influence the working point of the magnetron only to a small degree, and that the described favourable working condition is maintained with loads of the most difierent kinds.

What is claimed is:

1. A microwave oven comprising a wall structure defining a heating cavity, an opening in one wall of said cavity for supplying microwave energy thereto, a magnetron, a waveguide interconnecting said magnetron with said wall opening, a reactive matching element positioned within said waveguide between the magnetron and the wall opening, and a field stirrer rotatably mounted in said cavity in front of and close to said wall opening so as to reflect a portion of the microwave energy radiated from said opening back towards said magnetron, the reflected wave varying as a function of the movement of the stirrer, said reactive element being located within said waveguide and dimensioned relative to said stirrer so that the reflected waves from said matching element and stirrer produce a resultant wave in the direction towards the magnetron having a given phase variation that corresponds to operation of the magnetron in the sink region of its characteristic diagram, said phase variation producing a substantial variation in the magnetron frequency thereby to improve the field distribution within .said heating cavity.

2. An oven as described in claim 1 wherein said reactive matching element com-prises a metal member that is adjustable in the length direction of the waveguide.

3. A microwave oven comprising a wall structure defining a heating cavity, one wall of said cavity having an opening therein for supplying microwave energy to said cavity, a magnetron, a waveguide interconnecting said magnetron with said wall opening, a reactive matching element positioned within said waveguide between the magnetron and the wall opening, and variable reactance means comprising a field stirrer rotatably mounted in said cavity adjacent to said wall opening and interposed between said opening and the cavity heating load, said stirrer being arranged to reflect a portion of the radiated microwave energy back towards said magnetron to produce a reflected wave exhibiting a given phase variation which is a function of the position of said stirrer, the position of said matching element within the waveguide and the dimensions thereof being chosen relative to said stirrer so that the reflected waves from said matching element and stirrer produce a resultant wave in said waveguide having a given phase variation, said given phase variation of the resultant wave corresponding to operation of the magnetron in its sink region whereby a substantial variation in the magnetron frequency is produced as a function of the variable reactance of said stirrer.

4. An oven as described in claim 3 wherein said reactive matching element comprises a metal pin movably mounted in a longitudinal .slot in one side wall of the waveguide.

5. A microwave oven comprising a wall structure defining a heating cavity, one wall of said cavity having an opening therein for supplying microwave energy to said cavity, a magnetron, a waveguide interconnecting said magnetron with said wall opening, and means for mismatching said magnetron and waveguide to produce a periodically varying reflected wave in which the phase of the reflected wave corresponds to operation of the magnetron in its sink region over the whole variation range of said reflected wave, said mismatching means comprising a rotatable field stirrer mounted entirely within said cavity and opposite said wall opening so as to reflect a portion of the radiated energy back towards said magnetron to produce a reflected wave in the waveguide which varies in accordance with the displacement of the stirrer, and an adjustable reactive matching element positioned in said waveguide relative to said stirrer and dimensioned relative thereto so as to cooperate with said stirrer to produce said periodically varying wave.

References Cited UNITED STATES PATENTS 3,218,429 11/1965 Lenart 2l9-10.55

FOREIGN PATENTS 1,378,280 10/1964 France.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

1. A MICROWAVE OVEN COMPRISING A WALL STRUCTURE DEFINING A HEATING CAVITY, AN OPENING IN ONE WALL OF SAID CAVITY FOR SUPPLYING MICROWAVE ENERGY THERETO, A MAGNETRON, A WAVEGUIDE INTERCONNECTING SAID MAGNETRON WITH SAID WALL OPENING, A REACTIVE MATCHING ELEMENT POSITIONED WITHIN SAID WAVEGUIDE BETWEEN THE MAGNETRON AND THE WALL OPENING, AND A FIELD STIRRER ROTATABLY MOUNTED IN SAID CAVITY IN FRONT OF AND CLOSE TO SAID WLL OPENING SO AS TO REFLECT A PORTION OF THE MICROWAVE ENERGY RADIATED FROM SAID OPENING BACK TOWARDS SAID MAGNETRON, THE REFLECTED WAVE VARYING AS A FUNCTION OF THE MOVEMENT OF THE STIRRER, SAID REACTIVE ELEMENT BEING LOCATED WITHIN SAID WAVEGUIDE AND DIMENSIONED RELATIVE TO SAID STIRRER SO THAT THE REFLECTED WAVES FROM SAID MATCHING ELEMENT AND STIRRER PRODUCE A RESULTANT WAVE IN THE DIRECTION TOWARDS THE MAGNETRON HAVING A GIVEN PHASE VARIATION THAT CORRESPONDS TO OPERATION OF THE MAGNETRON IN THE SINK REGION OF ITS CHARACTERISTIC DIAGRAM, SAID PHASE VARIATION PRODUCING A SUBSTANTIAL VARIATION IN THE MAGNETRON FREQUENCY THEREBY TO IMPROVE THE FIELD DISTRIBUTION WITHIN SAID HEATING CAVITY. 